xidative processes are a major contributing factor in senile cataracts. We have demonstrated that metal-catalyzed oxidation of the crystallins induces protein modifications that mimic those seen in aging, senile cataracts, and brunescent lenses. The importance of the role of metal in oxidative processes related to cataract is further supported by studies in other laboratories. The lens contains high levels of ascorbate and thiols, such as glutathione, that can participate in these metal-catalyzed oxidation reactions. Our laboratory has continued studies on the mechanisms that protect the lens against deleterious oxidation reactions. A protein that protects enzymes specifically against inactivation by thiol-dependent, metal-catalyzed reactions is present in lenses of bovine, guinea pig, human, pig, monkey, and rat. (This activity is not related to catalase, glutathione peroxidase, or superoxide dismutase.) The antioxidant activity consistently copurifies with a subpopulation of glutathione S-transferase mu. To further understand the role of glutathione S-transferase in protecting against oxidative stress, the bovine enzyme was cloned. Three independent clones were sequenced, yielding the complete sequence of the protein. Two of the clones were expressed in Escherichia coli. The recombinant glutathione S-transferases are both active transferases. They also provide protection against thiol-dependent, metal-catalyzed oxidation of enzymes but not against ascorbate-dependent, metal-catalyzed reactions.